1. Field of the Invention
The present invention relates to an internal combustion engine, a vehicle, a marine vessel, and an exhaust gas cleaning method for an internal combustion engine.
2. Description of the Related Art
Conventionally, internal combustion engines including a three-way catalyst for cleaning exhaust gas as proposed in Japanese Laid-Open Patent Publication No. 07-205890 are known.
In order to maximize the cleaning efficiency of the three-way catalyst, the air/fuel ratio of the mixed gas (hereinafter, referred to as the “combustion air/fuel ratio”) needs to be the theoretical air/fuel ratio. However, when the internal combustion engine is driven at the theoretical air/fuel ratio in a high load state, the combustion temperature becomes excessively high, and also the temperature of the exhaust gas becomes high. When the temperature of the exhaust gas becomes excessively high, the catalyst is exposed to an excessively high temperature and is deteriorated due to the phenomenon of so-called sintering. As a result, the cleaning efficiency of the catalyst is decreased. For this reason, in order to avoid the temperature of the exhaust gas from becoming excessively high, the internal combustion engine as proposed in Japanese Laid-Open Patent Publication No. 07-205890 is driven at an air/fuel ratio smaller than the theoretical air/fuel ratio when in a high load state. Therefore, the cleaning performance of the three-way catalyst cannot be utilized to the maximum possible degree.